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Cymothoa exigua, or the tongue-eating louse, is a parasitic crustacean of the family Cymothoidae. This parasite enters fish through the gills, and then attaches itself at the base of the fish's tongue. The female attaches to the tongue and the male attaches on the gill arches beneath and behind the female. Females are 8–29 millimetres (0.3–1.1 in) long and 4–14 mm (0.16–0.55 in) in maximum width. Males are approximately 7.5–15 mm (0.3–0.6 in) long and 3–7 mm (0.12–0.28 in) wide. It extracts blood through the claws on its front, causing the tongue to atrophy from lack of blood. The parasite then replaces the fish's tongue by attaching its own body to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue. It appears that the parasite does not cause any other damage to the host fish. Once C. exigua replaces the tongue, some feed on the host's blood and many others feed on fish mucus. This is the only known case of a parasite functionally replacing a host organ. There are many species of Cymothoa, but only C. exigua is known to consume and replace its host's tongue.

DistributionThe distribution of Cymothoa exigua is quite widespread. It can be found from the Gulf of California south to north of the Gulf of Guayaquil, Ecuador. Two host records were also recently discovered in Costa Rica. It has been sampled in waters from 2 metres (6 ft 7 in) to almost 60 m (200 ft) deep. This isopod is known to parasitize eight species in two orders and four families of fishes [7 species of order Perciformes: 3 snappers (Lutjanidae), 1 grunt (Haemulidae), 3 drums (Sciaenidae), and 1 species of order Atheriniformes: 1 grunion (Atherinidae)]. Females of this isopod were found in the mouths of three species of snappers. New hosts from Costa Rica include the Colorado snapper, Lutjanus colorado and Jordan's snapper, L. jordani. In 2005, a fish parasitised by what could be Cymothoa exigua was discovered in the United Kingdom. As the parasite is normally found off the coast of California, this led to speculation that the parasite's range may be expanding; however, it is also possible that the isopod traveled from the Gulf of California in the snapper's mouth, and its appearance in the UK is an isolated incident.

Life-CycleNot much is known about the life cycle of C. exigua. It exhibits sexual reproduction. It is likely that juveniles first attach to the gills of a fish and become males. As they mature, they become females, with mating likely occurring on the gills. If there is no female present, within a pair of two males, one male can turn into a female after it grows to 10mm in length. The female then makes its way to the fish's mouth where it uses its front claws to attach to the fish’s tongue.

Relationship With HumansIt is currently believed that C. exigua are not harmful to humans unless picked up alive, in which case they can bite. In Puerto Rico, C. exigua was the leading subject of a lawsuit against a large supermarket chain. Because C. exigua is found in snappers from the Eastern Pacific and is shipped worldwide for commercial consumption, contamination of the parasite is inevitable. The customer in the lawsuit claimed to have been poisoned by eating an isopod cooked inside a snapper. This case, however, was dropped stating that isopods are not poisonous to humans and some are even consumed as part of a regular diet. C. exigua, in a mutated form, were featured as the villains in the 2012 Barry Levinson film The Bay.